This invention relates to an apparatus and method for controlling the venting of vapor from a vessel.
The invention is particularly applicable to refrigeration systems in which, as will become more apparent below, controlled venting of vapor from a refrigerant-containing vessel is desirable.
Commercial refrigeration systems employed in refineries, for example, typically operate by at least partially vaporizing a liquid refrigerant such as propane to effect cooling, compressing the vaporized refrigerant, and then condensing the compressed refrigerant. In a closed system, the condensed product can then be passed to a vessel, sometimes called a surge vessel, in which the refrigerant exists in both a liquid phase and a vapor phase.
In such refrigeration systems it is usually cheaper and thus desirable to use an impure refrigerant such as commercial grade propane which contains impurities (i.e., ethane). Some problems arise with the use of such impure refrigerants, however. Most importantly, impurities cause an undesirably high vapor pressure against which the compressor must compress, thus leading to a waste of energy in compression. It then becomes necessary to vent vapor from the surge vessel in order to reduce the vapor pressure of the refrigerant.
The oldest and simplest method of venting a surge vessel is manually by an operator who can simply monitor the vessel vapor pressure and open a vent valve when the pressure becomes too high. Such manual venting control is highly inefficient for removing some impurities. Thus, automatic techniques for venting surge vessels were developed. One such technique involves measuring the temperature of the vessel liquid phase, correlating the measured temperature with vapor pressure characteristics of a desired refrigerant composition to be contained in the vessel so as to obtain a desired vapor pressure corresponding to the desired composition, and controlling venting of vapor from the surge vessel in response to the results of the comparison between the actual vessel vapor pressure and the above obtained desired vapor pressure.
Even though prior automatic techniques for the controlled venting of vapor in refrigeration systems accomplish control of vessel vapor pressure to an adequate degree, improvement with respect to accuracy of control, efficiency and complexity of design would be desirable.